Method for detecting rheumatoid arthritis-specific autoantibodies

ABSTRACT

A method for detecting anti-filaggrin-autoantibodies specific to rheumatoid arthritis in a biological sample, by providing a filaggrin with no citrulline residues (FNC) having an arginine residue or a filaggrin peptide with no citrulline residues (PFNC) having an arginine residue. Also provided are a citrulline-containing filaggrin (FC) having the peptide sequence of the FNC where some arginine residues have been converted to citrulline residues or a citrulline-containing peptide (PFC) having the peptide sequence of the PFNC, where at least one arginine residue has been converted to a citrulline residue. The sample is contacted with the FNC or PFNC and the FC or PFC, and the resulting immune complexes are detected and quantified, with a value X NC  and X C , respectively. Anti-filaggrin-autoantibodies specific to rheumatoid arthritis are determined to be present in the biological sample if the value of X C  is greater than the value of X NC .

Rheumatoid arthritis (RA) is the most common of the chronic inflammatory rheumatisms. It is a systemic autoimmune disease characterized by an inflammation of the joints, which affects more than 500 000 individuals in France and more than 2 million in the United States. The serum of affected patients contains autoantibodies, some of which are specific and may constitute a marker for the disease allowing diagnosis thereof even at an early stage.

Autoantibodies which are specifically present in patients suffering from RA and which react with a rat esophageal epithelial antigen were described for the first time by B. J. J. Young et al., in Br. Med. J. 2:97-99 (1979). At the time, these autoantibodies were named “anti-keratin antibodies” (AKAs), but it has subsequently been shown that these antibodies are in fact antifilaggrin autoantibodies (AFAs). The antigens for these antibodies are the neutral/acidic forms of filaggrins (Simon et al., J. Clin. Invest., 92, 1387, (1993)).

Filaggrin is generated from profilaggrin, which is a phosphorylated polyprotein of epidermal keratohyalin granules. Profilaggrin has a high molecular weight (approximately 400 000 in humans), and it is soluble in the presence of high concentrations of salts or of urea. It has a high content of basic amino acids (arginine and histidine), and also of glycine, serine and glutamic acid. It has a low content of nonpolar amino acids and contains neither methionine, nor cysteine, nor tryptophan. It is highly phosphorylated on serine residues, which confers on it an isoelectric point to close to neutrality.

Profilaggrin is cleaved into filaggrin units in the course of a complex process of maturation involving dephosphorylation followed by cleavage by proteases in the interdomain segments. This cleavage first generates fragments of intermediate size, and then the functional filaggrin molecules.

Filaggrins derived from the dephosphorylation and cleavage of profilaggrin are basic proteins, the amino acid content of which is similar to that of the profilaggrins. They contribute to keratin filament organization and undergo gradual maturation during which the arginine residues, which are basic, are converted to citrulline residues, which are neutral, under the action of peptidylarginine deiminase (Harding C. R. and Scott I. R., J. Mol. Biol. 170, p. 651-673 (1983)). This leads to a decrease in their affinity for the keratins, from which they detach; they are then completely degraded by various proteases.

The properties of filaggrins and of pro-filaggrins have been particularly well studied in rats, in mice and in humans. The size of profilaggrin varies, according to species, from 300 to 400 kD, and that of the filaggrins from 27 to 64 kD.

The gene encoding profilaggrin is made up of repeated subunits, each one of which encodes a filaggrin molecule, separated by portions encoding the interdomain peptide segments. All the repeat units encoding each of the human filaggrins have the same length (972 base pairs in humans); however, in humans, considerable sequence variations (10-15%) are observed from one subunit to the other. While most are conservative, some of these variations induce amino acid changes and, in some cases, changes in the electric charge of the protein. Thus, human filaggrins form, independently of the post-transcriptional modifications, a heterogeneous population of molecules of similar size but of different sequences and charges (pHi equal to 8.3±1.1) (Gan et al., Biochem. 29, p. 9432-9440 (1990)).

The polymorphism observed in humans between the sequences of the filaggrin units within the same profilaggrin gene is not apparent in rats and mice. Filaggrins also exhibit a great inter- and intraspecific variability in terms of their sequence. This variability does not however affect their functional properties, nor their overall amino acid composition or their biochemical properties. Similarly, the tissue localizations of profilaggrin and of filaggrins are identical in the various mammals studied.

The autoantibodies (AFAs) are associated with the clinical and biological parameters which define the most active and the most severe forms of the disease, and are thought to precede the appearance of the clinical signs. The detection thereof therefore proves to be an essential step in the diagnosis of RA.

According to document WO-A-98/08946 in the applicant's name, a method has been developed for detecting AFAs, in a biological sample, based on the discovery that citrullination of filaggrin is necessary for detection of AFAs present in sera from patients suffering from RA, this citrullination generating epitopes recognized by the AFAs. This method comprises the following steps:

the biological sample analyzed is brought into contact with a citrulline-containing recombinant filaggrin or a fragment thereof, under conditions suitable for the formation of immune complexes with the AFAs, and

the immune complexes possibly formed with the AFAs are detected.

The results demonstrate that the method is highly sensitive, most of the sera from patients suffering from RA recognizing the citrulline-containing recombinant filaggrin.

It was however observed, in the course of the steps for diagnosing RA in the patients whose sera were found to be positive in this method, that not all were RA cases. This method in fact leads to the detection of false positives, as will be illustrated in the following examples, and cannot therefore be used reliably, due to an insufficient specificity.

The present inventors have now developed a new ELISA assay for diagnosing RA in patients, which has great specificity while at the same time conserving the high sensitivity of the abovementioned method.

According to this method, rheumatoid arthritis (RA)-specific autoantibodies (AFAs) are detected in a biological sample liable to contain said autoantibodies (AFAs) and other autoantibodies not specific for RA, as follows:

firstly, a filaggrin (FNC) or a derivative of filaggrin (FNC) or a filaggrin peptide (PFNC) comprising at least one arginine residue and, secondly, said citrulline-containing filaggrin (FC) or a derivative of said citrulline-containing filaggrin (FC) or said citrulline-containing peptide (PFC) are provided,

said biological sample is brought into contact with, firstly, said filaggrin (FNC) or said derivative of said filaggrin (FNC) or said peptide (PFNC) and, secondly, said filaggrin (FC) or said derivative of said filaggrin (FC) or said peptide (PFC), under conditions suitable for the formation of immune complexes with the autoantibodies (AFAs),

the immune complexes formed between the auto-antibodies (AFAs) or the other antibodies present in the sample and, firstly, said filaggrin (FNC) or said derivative of said filaggrin (FNC) or said peptide (PFNC) and, secondly, said citrulline-containing filaggrin (FC) or said derivative of said citrulline-containing filaggrin (FC) or said peptide (PFC) are detected and quantified, this quantification being expressed by a value respectively X_(NC) and X_(C),

the value of X_(C) is reduced by the value of X_(NC).

To implement the method of the invention, the filaggrin (FNC) or the derivative of a filaggrin (FNC) or the peptide (PFNC) is immobilized on the solid support and the filaggrin (FC) or a derivative of the filaggrin (FC) or the peptide (PFC) is immobilized on another solid support. They may be, inter alia, two wells on the microtitration plate or two tips of the Vidas device (registered trademark) marketed by the applicant, but also strips or any other suitable support.

Before describing the invention in greater detail and presenting the preferred variants thereof, some terms used in the description and the claims are defined below.

Citrullination is the reaction of conversion of an arginine residue to a citrulline residue by conversion of the —C(NH)NH₂ group of the arginine to —CONH₂. The reaction is advantageously carried out in vitro with the peptidylarginine deiminase (PAD) enzyme. This enzyme may be a rabbit muscle PAD; it is however within the scope of those skilled in the art to select another suitable PAD. It is of course possible to envision any other means of citrullination, such as chemical reagents or microorganisms.

According to the invention, when reference is made to a citrulline-containing filaggrin, this means that a proportion of approximately at least 20% of the arginine residues have been converted to citrulline residues, this conversion taking place randomly. Preferably, this degree of citrullination is at least 30%, and advantageously at least 50%. It has been observed that the immunoreactivity of a citrulline-containing filaggrin for which the degree of citrullination is approximately 80% is similar to that of a filaggrin for which the degree of citrullination is approximately 50%. Under these conditions, it is therefore unnecessary to seek to achieve very high degrees of citrullination.

If the citrullination is not carried out randomly, but is region-specific, and in particular if the conversion reaction can be limited to an arginine residue belonging to an epitope of the filaggrin after citrullination, the degree of citrullination can be lower than the abovementioned values, without the immunoreactivity of the filaggrin thus citrullinated being effected by this.

The expression “derivative of a filaggrin (FNC)” or “corresponding citrulline-containing derivative” means: a chimeric filaggrin which consists of a combination of fragment(s) of sequences of human and rat filaggrin and/or a combination of fragment(s) of sequences of human and/or rat filaggrin and of consensus peptide(s).

To implement the method of the invention, the filaggrin is preferably chosen from human filaggrin and a filaggrin of animal origin. It advantageously consists of a recombinant rat filaggrin having the sequence SEQ ID No. 1.

In place of the filaggrin, it is possible to choose a filaggrin peptide (PFNC), the sequence of which comprises at least one arginine residue, and the corresponding citrulline-containing peptide (PFC) for which said arginine residue at least of PFNC has been converted to a citrulline residue. According to a variant of the method of the invention, the peptide contains an epitope of filaggrin after citrullination. Thus, a peptide (PFNC) may in particular be chosen from the following peptides:

-   -   the S47S peptide, the sequence of which is represented in SEQ ID         No. 2, corresponding to amino acids 71 to 119 of the sequence of         a human filaggrin unit and containing 6 arginine residues,     -   the E12H peptide, the sequence of which is represented in SEQ ID         No. 3, as determined with reference to the nucleotide sequences         of the human profilaggrin gene described by Gan S. Q. et al.         (Biochemistry, 29: 9432-9440, (1990)) and comprising 1 arginine         residue,     -   the E12D peptide, the sequence of which is represented in SEQ ID         No. 4, as determined with reference to the nucleotide sequences         of the human profilaggrin gene described by Gan S. Q. et al.         (Biochemistry, 29: 9432-9440, (1990)) and comprising 3 arginine         residues,     -   the G22Q peptide, the sequence of which is represented in SEQ ID         No. 5, corresponding to a consensus sequence of a rat filaggrin         unit comprising 4 arginine residues,     -   the G26E peptide, the sequence of which is represented in SEQ ID         No. 6, corresponding to a consensus sequence of a rat filaggrin         unit comprising 10 arginine residues.

As mentioned above, while a filaggrin (FNC) is provided for implementing the method of the invention, the citrulline-containing filaggrin (FC) used has the peptide sequence of filaggrin (FNC), in which at least 20%, preferably at least 30%, and better still at least 50%, of the arginine residues of said FNC are converted to citrullines.

The autoantibodies (AFAs) are circulating antibodies, and a biological sample to be analyzed according to the method of the invention may be chosen from blood, plasma and serum.

The detection and quantification of the immune complexes formed are carried out by any techniques well known to those skilled in the art, and in particular using an enzyme for the detection and its calorimetric or fluorescent substrate for the quantification (examples: alkaline phosphatase/4-methylumbelliferyl phosphate, peroxidase/ortho-phenylenediamine).

As will be illustrated in the examples, it is thus possible to bring the immune complexes formed into contact with a conjugate consisting of a labeled antibody directed against human immunoglobulins, under conditions suitable for the formation of labeled immune complexes, and then the labeled immune complexes are detected and quantified. The conjugate is advantageously an anti-human immunoglobulin antibody, whole or fragmented (examples: Fab′, Fab′₂ fragments), labeled with peroxidase or with alkaline phosphatase, and the formation of labeled immune complexes is then detected and quantified by colorimetry or fluorimetry, the quantification being expressed as optical density or fluorescence values X_(NC) and X_(C).

A positive assay revealing the presence of RA-specific autoantibodies (AFAs) in the sample analyzed is characterized by the value X_(C) being greater than the value X_(NC).

The invention is illustrated below and its characteristics and advantages are brought to light.

EXAMPLE 1 Production of a Recombinant Rat Filaggrin

A) Cloning of the Rat Profilaggrin Gene

The studies by Haydock and Dale, 1986 (J. Biol. Chem., 261, 12520-12525) and by Rothnagel et al., 1987 (J. Biol. Chem., 262, 15643-15648) have shown that the rat profilaggrin gene is made up of 20±2 repeat units of approximately 1200 base pairs (bp) and has no intron in its coding region, each profilaggrin unit being made up of a filaggrin domain and a binding sequence.

Furthermore, the coding sequence for two incomplete profilaggrin units has been published by Haydock and Dale, 1990 (DNA Cell Biol., 9, 251-261).

Based on these data, oligonucleotides intended for the PCR amplification of the coding sequence for a complete rat profilaggrin unit were designed. The rat genomic DNA matrix was prepared from whole blood of a Wistar rat. Three different PCR products were obtained with three different pairs of oligonucleotides. These PCR products were cloned into a vector pCAPs (PCR cloning kit, Boehringer) and sequenced. They exhibit a high degree of homology (of more than 99%) with one another and also a 95% and 98% homology with the two incomplete profilaggrin units published by Haydock and Dale, 1990.

These data confirm the nature of the DNA obtained and suggest that the available sequences are representative of the existing rat filaggrin domains, a certain inter- and intra-individual variability being expected.

One of the sequences obtained was cloned into an expression vector pMR (Cheynet et al., 1993, Prot. Exp. Purif., 4, 367-372) as a fusion with a coding sequence for 6 histidines in the 5′ position. The construct identified as pOL041 was used to transform E. coli bacteria. It encodes a 399 amino acid protein with a theoretical molecular weight of 42 210, having the sequence SEQ ID No. 1.

B) Expression and Purification of the Recombinant Rat Filaggrin

Culturing:

To place the transformed E. coli strain (BL21 or DH5α) in culture, an isolation from the library conserved at −80° C. is carried out on agar medium with 0.1 mg/ml ampicillin.

From an isolated colony, a preculture is prepared in liquid medium (yeast extract, 2% glucose and 0.1 mg/ml ampicillin) in Erlenmeyer flasks placed at +37° C. and with orbital shaking (280 rpm).

This preculture then makes it possible to seed 5 l Erlenmeyer culture flasks, each containing 1 l of the same medium optionally substituted with PMSF at a final concentration of 0.5 mM. These Erlenmeyer flasks are cultured at +37° C. with shaking at 280 rpm, until an OD at 600 nm of 0.8 is obtained. Once this OD value has been reached, the culturing is continued for a period of 3 hours.

The culture is then stopped and centrifuged for 25 minutes at 6-000 g. The bacterial pellets are then collected and conserved at −25° C.

Purification:

The bacterial pellet is lysed by the action of lysozyme in the presence of protease inhibitor in a buffer consisting of 0.1 M NaH₂PO₄/0.6 M NaCl/0.2% Tween/1 mM Na azide/6 mM imidazole, pH 7.3, and by sonication. This lysate is then clarified for 30 minutes at 23 700 g and the supernatant is conserved.

The recombinant filaggrin is purified by metal (nickel) chelate affinity chromatography at +4° C.

Briefly, after re-equilibration of the gel in lysis buffer, the lysis supernatant is injected onto the column, followed by washing (and monitoring of the UV 280 nm detection signal) with the buffer consisting of 0.1 M NaH₂PO₄/0.6 M NaCl/1 mM Na azide/6 mM imidazole, pH 7.3, until the UV 280 nm detection signal has returned to base line.

The recombinant filaggrin (rFlg), adsorbed onto the gel, is eluted with the buffer consisting of 0.1 M NaH₂PO₄/0.6 M NaCl/1 mM Na azide/0.5 M imidazole, pH 7.15.

The fraction thus obtained is concentrated on a 10 kDa membrane.

The final purification step is carried out by gel filtration chromatography on a Superdex 200 support, in 20 mM Tris/0.3 M NaCl buffer, pH 11. The concentrate is injected onto the column, the fractions being selected by monitoring the UV signal at 280 nm and 214 nm. This step makes it possible to isolate and recover the immunologically active recombinant filaggrin of interest, in the second elution peak (MW≈42 000).

EXAMPLE 2 Citrullination of the Recombinant Filaggrin

After assaying the total proteins by the Bradford method (Biorad reagents), the citrullination is performed, by the action of peptidylarginine deiminase, for 4 hours at +37° C. with magnetic stirring, at a rate of 4 enzyme units/mg of total proteins, in the presence of 0.1 M EDTA/1 M DTT/0.5 M CaCl₂. The amount of total proteins is then re-assayed on the deiminated fraction.

The citrulline-containing filaggrin is purified by Ni-NTA chromatography and then gel filtration chromatography.

The degree of citrullination detected is 53%.

Examples 3 and 4 below illustrate the diagnosis of RA in a method of detection of the prior art and in a method of detection according to the invention, so as to allow the comparison thereof.

The method of detection of the prior art consists of an ELISA assay, of the type such as that described in WO-98/08946 in the applicant's name, using citrulline-containing filaggrin and based on the detection of immune complexes formed between the AFA antibodies possibly present in the sample and the citrulline-containing filaggrin.

The method of detection according to the invention is an ELISA assay using citrulline-containing filaggrin and non-citrulline-containing filaggrin and which is based on the parallel detection of the immune complexes formed, firstly, with the citrulline-containing filaggrin and, secondly, the non-citrulline-containing filaggrin, and on the difference in values between the two.

EXAMPLE 3

A) Samples

The assays were carried out on the following samples:

63 control sera affected by the following pathological conditions: algodystrophy, gout, malignant hemopathy, Paget's disease, neuralgia, mechanical pathology, periarteritis nodosa, polymyositis, lupus-related rheumatism, psoriatic rheumatism, scleroderma, ankylosing spondylitis, spondylarthropathy, Gougerot's syndrome, Sharp's syndrome, vasculitis.

65 sera from patients suffering from clinically diagnosed rheumatoid arthritis.

B) Material

Nunc Maxisorp 468667 plate

Incubator at 35-39° C.

Axia microwasher U4402

Biotek (commercial trademark) plate reader

C) Protocol

Deposition in wells: 100 μl per well of 2.5 μg/ml of filaggrin for 2 hours at 37° C. in PBS buffer, pH 7.2;

3 washed with 300 μl per well of PBS/0.05% Tween buffer, pH 7.2;

blocking for 1 hour at 37° C. with 250 μl per well of PBS/1% BSA buffer, pH 7.2;

3 washed with 300 μl per well of PBS/0.05% Tween buffer, pH 7.2;

Incubation for 1 hour at 37° C. with 100 μl per well of serum diluted to 1/100 in buffer consisting of 10 mM Tris, 350 mM NaCl, 1% TritonX100, 1% BSA, 10% rabbit serum, pH 7.6;

3 washed with 300 μl per well of PBS/0.05% Tween buffer, pH 7.2;

incubation of an anti-human IgG conjugate diluted in buffer consisting of 10 mM Tris, 350 mM NaCl, 1% TritonX100, 1% BSA, 10% rabbit serum, pH 7.6; 100 μl per well;

3 washed with 300 μl per well of PBS/0.05% Tween buffer, pH 7.2.

Enzymatic Revelation:

-   -   100 μl of ortho-phenylenediamine (OPD) per well     -   incubation for 10 min at 18-25° C.     -   blocking with 100 μl H₂SO₄     -   reading at 492 nm.

The samples are assayed in parallel on a plate coated with non-citrulline-containing filaggrin and on another plate coated with citrulline-containing filaggrin.

The samples are assayed unduplicated.

The results are expressed as optical density. The signals indicated at an optical density of 3 are equal to or greater than 3.

D) Results

The results are given in tables 1 and 2 below.

With the results obtained on citrulline-containing filaggrin according to WO-A-98/08946 (table 1), the performance levels are as follows:

100% specificity threshold   34% sensitivity (22/65) at 1.337  98% specificity threshold   38% sensitivity (22/65) at 0.937  95% specificity threshold 61.5% sensitivity (40/65) at 0.335

With the results obtained on citrulline-containing filaggrin/non-citrulline-containing filaggrin according to the invention (table 2), the performance levels are as follows:

100% specificity threshold 38% sensitivity (25/65) at 0.731  98% specificity threshold 69% sensitivity (45/65) at 0.009  95% specificity threshold 77% sensitivity (50/65) at −0.004

In both cases, at 95% specificity, 3 sera are false positives.

All the samples detected on the citrulline-containing filaggrin phase are detected on the differential system with 100% and 98% specificity. A single serum is additionally detected with the differential system. At 95% specificity, 7 samples are additionally detected with the differential system.

The differential system is more effective than the citrulline-containing filaggrin phase on this population, with 69% sensitivity for 98% specificity.

For this population, the performance levels of the assays normally used are as follows:

GIFOR (immunofluorescence on section of rat esophagus)

98% specificity threshold 49% sensitivity (32/65) at 2 Gblot 98% specificity threshold 58% sensitivity (38/65) at 2.25

The performance levels obtained by ELISA on rat filaggrin are better than the techniques currently used, GIFOR and GBlot.

TABLE 1 Results as OD at 492 nm of the ELISA assays on citrulline-containing filaggrin (FC) Negative controls FC RA sera FC 58 0.050 78 0.051 57 0.056 76 0.059 10 0.058 72 0.077 33 0.060 74 0.077 51 0.062 81 0.079 53 0.062 79 0.088 44 0.064 73 0.091 54 0.066 71 0.098 47 0.069 80 0.102 49 0.069 75 0.109 38 0.070 65 0.113 39 0.070 86 0.121 40 0.071 66 0.124 50 0.072 83 0.132 29 0.073 69 0.141 45 0.073 77 0.147 25 0.076 84 0.151 55 0.076 85 0.156 28 0.079 90 0.157 42 0.083 67 0.158 46 0.083 68 0.158 60 0.085 70 0.202 43 0.087 89 0.207 31 0.088 88 0.276 34 0.088 92 0.299 37 0.090 93 0.387 41 0.090 87 0.409 16 0.092 64 0.434 35 0.092 82 0.472 48 0.092 95 0.496 26 0.093 98 0.499 52 0.093 96 0.524 17 0.097 94 0.535 61 0.098 100 0.611 27 0.111 99 0.659 19 0.113 91 0.667 20 0.113 103 0.726 4 0.116 101 0.729 22 0.116 102 0.801 36 0.123 97 0.853 18 0.124 104 0.939 24 0.124 105 1.203 14 0.128 106 1.240 12 0.129 107 1.526 23 0.130 108 1.694 7 0.133 109 1.900 5 0.134 110 2.076 9 0.135 111 2.387 30 0.142 114 2.593 59 0.146 112 2.610 6 0.158 113 3.000 21 0.159 115 3.000 11 0.166 116 3.000 32 0.167 117 3.000 15 0.169 118 3.000 62 0.193 119 3.000 56 0.222 120 3.000 3 0.253 121 3.000 13 0.269 122 3.000 1 0.335 123 3.000 8 0.369 124 3.000 63 0.937 125 3.000 2 1.337 126 3.000 127 3.000 128 3.000

TABLE 2 Results as OD at 492 nm of the ELISA assays on citrulline-containing filaggrin (FC) and non- citrulline-containing filaggrin (FNC) Negative controls FNC FC FC − FNC RA sera FNC FC FC − FNC 1 0.780 0.335 −0.445 64 0.777 0.434 −0.343 2 1.747 1.337 −0.410 65 0.202 0.113 −0.089 3 0.433 0.253 −0.180 66 0.204 0.124 −0.080 4 0.268 0.116 −0.152 67 0.222 0.158 −0.064 5 0.271 0.134 −0.137 68 0.221 0.158 −0.063 6 0.294 0.158 −0.136 69 0.195 0.141 −0.054 7 0.262 0.133 −0.129 70 0.255 0.202 −0.053 8 0.492 0.369 −0.123 71 0.131 0.098 −0.033 9 0.231 0.135 −0.096 72 0.109 0.077 −0.032 10 0.149 0.058 −0.091 73 0.115 0.091 −0.024 11 0.256 0.166 −0.090 74 0.099 0.077 −0.022 12 0.218 0.129 −0.089 75 0.131 0.109 −0.022 13 0.351 0.269 −0.082 76 0.072 0.059 −0.013 14 0.209 0.128 −0.081 77 0.157 0.147 −0.010 15 0.248 0.169 −0.079 78 0.055 0.051 −0.004 16 0.158 0.092 −0.066 79 0.090 0.088 −0.002 17 0.162 0.097 −0.065 80 0.104 0.102 −0.002 18 0.188 0.124 −0.064 81 0.079 0.079 0.000 19 0.174 0.113 −0.061 82 0.466 0.472 0.006 20 0.173 0.113 −0.060 83 0.124 0.132 0.008 21 0.219 0.159 −0.060 84 0.137 0.151 0.014 22 0.171 0.116 −0.055 85 0.137 0.156 0.019 23 0.179 0.130 −0.049 86 0.101 0.121 0.020 24 0.172 0.124 −0.048 87 0.372 0.409 0.037 25 0.123 0.076 −0.047 88 0.236 0.276 0.040 26 0.139 0.093 −0.046 89 0.137 0.207 0.070 27 0.153 0.111 −0.042 90 0.057 0.157 0.100 28 0.120 0.079 −0.041 91 0.514 0.667 0.153 29 0.113 0.073 −0.040 92 0.058 0.299 0.241 30 0.181 0.142 −0.039 93 0.134 0.387 0.253 31 0.126 0.088 −0.038 94 0.240 0.535 0.295 32 0.204 0.167 −0.037 95 0.156 0.496 0.340 33 0.096 0.060 −0.036 96 0.122 0.524 0.402 34 0.122 0.088 −0.034 97 0.424 0.853 0.429 35 0.125 0.092 −0.033 98 0.063 0.499 0.436 36 0.156 0.123 −0.033 99 0.208 0.659 0.451 37 0.122 0.090 −0.032 100 0.117 0.611 0.494 38 0.101 0.070 −0.031 101 0.207 0.729 0.522 39 0.101 0.070 −0.031 102 0.165 0.801 0.636 40 0.098 0.071 −0.027 103 0.072 0.726 0.654 41 0.117 0.090 −0.027 104 0.142 0.939 0.797 42 0.110 0.083 −0.027 105 0.231 1.203 0.972 43 0.112 0.087 −0.025 106 0.087 1.240 1.153 44 0.089 0.064 −0.025 107 0.118 1.526 1.408 45 0.097 0.073 −0.024 108 0.268 1.694 1.426 46 0.107 0.083 −0.024 109 0.136 1.900 1.764 47 0.092 0.069 −0.023 110 0.095 2.076 1.981 48 0.113 0.092 −0.021 111 0.127 2.387 2.260 49 0.089 0.069 −0.020 112 0.256 2.610 2.354 50 0.091 0.072 −0.019 113 0.555 3.000 2.445 51 0.081 0.062 −0.019 114 0.128 2.593 2.465 52 0.112 0.093 −0.019 115 0.518 3.000 2.482 53 0.081 0.062 −0.019 116 0.338 3.000 2.662 54 0.083 0.066 −0.017 117 0.315 3.000 2.685 55 0.092 0.076 −0.016 118 0.295 3.000 2.705 56 0.233 0.222 −0.011 119 0.224 3.000 2.776 57 0.065 0.056 −0.009 120 0.184 3.000 2.816 58 0.058 0.050 −0.008 121 0.184 3.000 2.816 59 0.153 0.146 −0.007 122 0.181 3.000 2.819 60 0.089 0.085 −0.004 123 0.171 3.000 2.829 61 0.095 0.098 0.003 124 0.160 3.000 2.840 62 0.184 0.193 0.009 125 0.145 3.000 2.855 63 0.206 0.937 0.731 126 0.081 3.000 2.919 127 0.062 3.000 2.938 128 0.056 3.000 2.944

EXAMPLE 4

This example makes it possible to compare a diagnostic test according to the invention, the ArFA-ELISA assay, and several diagnostic tests of the prior art which are commercially available or which use known methods, namely the AKA by immunofluorescence, AhFA-IB, AhFA-ELISA and CCP-ELISA assays.

A) Samples

The assays were carried out on 711 sera, 240 of which came from patients suffering from rheumatoid arthritis (RA) and 471 of which came from non-RA patients. Among the non-RA patients, 157 were suffering from an inflammatory disease chosen from progressive lupus erythematosus (21), systemic scleroderma (9), psioratic rheumatism (43), ankylosing spondylitis (40) and other inflammatory diseases (44), and 314 were suffering from a non-inflammatory disease chosen from arthrosis (104), compressive neuropathy (25), Paget's disease (68), reflex sympathetic dystrophy (29), serious bone diseases (49) and other non-inflammatory diseases (39).

The filaggrin is a recombinant rat filaggrin; the non-citrulline-containing filaggrin is obtained in accordance with example 1, and the citrulline-containing filaggrin is obtained by citrullination (40% degree of citrullination of arginine residues) of the non-citrulline-containing filaggrin in accordance with example 2.

B) Protocol

The detection of the antibodies according to the AKA by immunofluorescence and AhFA-IB assays was carried out according to the methods described in C. Vincent et al., Ann. Rheum. Dis. (1989) 48, 712-722, and C. Vincent et al., J. Rheumatol. (1998) 25, 838-846.

The AhFA-ELISA assay is used according to the method described by L. Nogueira et al., Ann. Rheum. Dis (2001) 60, 882-887.

The CCP-ELISA assay is carried out with the Immunoscan™ RA material (Euro-diagnostica, Arnhem, The Netherlands) used according to the manufacturer's recommendations.

The ArFA-ELISA assay according to the invention was used in accordance with protocol C) described in example 3 with the exception of the fact that, in the present example, the samples were assayed in quadruplicate. The average variation between the assays for the same samples was less than 6%.

The results obtained for each of the assays are given in table 3 below.

TABLE 3 95% 98.5% 99% specificity specificity specificity ‘AKA’ 0.52 0.45 0.40 (0.45-0.58) (0.38-0.51) (0.34-0.46) AhFA-IB 0.59 0.48 0.37 (0.53-0.65) (0.42-0.55) (0.31-0.43) AhFA- 0.53 0.38 0.37 ELISA (0.46-0.59) (0.32-0.44) (0.31-0.43) CCP-ELISA nd 0.58 0.50 (0.52-0.65) (0.44-0.57) ArFA- 0.76 0.67 0.65 ELISA (0.70-0.81) (0.60-0.73) (0.59-0.71)

It emerges from this table that the assay according to the invention is the most reliable of the assays examined in the detection of RA. 

1. A method for diagnosing rheumatoid arthritis (RA) in a human patient, the method comprising: obtaining a biological sample from the patient, providing a filaggrin peptide that has no citrulline residues (PFNC) consisting of the sequence set forth in SEQ ID NO:5, providing a citrulline-containing peptide (PFC) having the peptide sequence of said PFNC, wherein at least one arginine residue has been converted to a citrulline residue, contacting a first portion of said biological sample with said PFNC under conditions suitable for the formation of immune complexes with anti-filaggrin-autoantibodies (AFAs), contacting a second portion of said biological sample with said PFC under conditions suitable for the formation of immune complexes with AFAs, detecting and quantifying the formation of immune complexes formed between antibodies present in the sample and said PFNC and PFC, this quantification being expressed by a value X_(NC) and X_(C), respectively, determining that AFAs specific to RA are present in the biological sample only if the value of X_(C) is greater than the value of X_(NC), and diagnosing the patient as having RA if it is determined that AFAs specific to RA are present in the biological sample.
 2. The method as claimed in claim 1, wherein said PFC is obtained by the action of peptidylarginine deiminase.
 3. The method as claimed in claim 1, wherein the biological sample is selected from the group consisting of blood, plasma and serum.
 4. The method as claimed in claim 1, wherein the immune complexes formed are brought into contact with a conjugate comprising a labeled antibody directed against human immunoglobulins, under conditions suitable for the formation of labeled immune complexes, and then the labeled immune complexes are detected and quantified.
 5. The method as claimed in claim 4, wherein the conjugate is an anti-human immunoglobulin antibody labeled with alkaline phosphatase or with peroxidase, and then the formation of labeled immune complexes is detected and quantified by colorimetry or fluorimetry, the quantification being expressed as optical density or fluorescence values X_(NC) and X_(C). 